1. Field of the Invention
The present invention relates to a micro actuator for controlling a focal depth and, more particularly, to a micro actuator for controlling a focal depth capable of precisely controlling a focal depth of a lens focusing light.
2. Description of the Background Art
In case of an optical information recording and reproducing apparatus using a rotary optical disk as a recording and reproducing medium, a distance between a record and reproduce layer of the optical disk and a lens of an optical pick-up device changes incessantly due to a flatness error of an optical disk, a mechanical vibration of a motor rotating the optical disk and an eccentric error. Such a change deteriorates a signal-to-noise ratio of a recording and reproducing optical signal, limits a size of readable and recordable bits, and causes a mis-reading and mis-recording. Thus, in order to input light accurately focused into a record and reproduce layer, a focusing servo device adjusting a focal point of an input light actively is used.
Recently, with the optical recording and reproducing medium, in one method, in order to extend a record capacity, a record density is increased by reducing the size of data bit. In addition, in other methods which have been developed and put to practical use, a record and reproduce layer is formed at both sides of the optical disk, or a plurality of layers are stacked at one surface of the optical disk and optical information is recorded to, reproduced from and rewritten to each layer.
Especially, with the optical disk formed by stacking a plurality of layers, a focal depth controlling function of changing a focal position should be demanded to allow a laser beam to focus into an arbitrary layer where optical information is recorded, reproduced and rewritten.
Meanwhile, with the optical disk having the multi-layer, the focal distance changes infinitesimally and incessantly even within a single layer due to the aforementioned disk flatness error, a mechanical vibration of the motor rotating the optical disk and an eccentric error, and a thickness error of multi-layers formed in the optical disk.
Therefore, in case of the optical recording and reproducing apparatus using the multi-layered optical disk as a recording and reproducing medium, it is preferred to provide a precise focal depth control function for correcting a fine focal depth in an individual layer, as well as the focal depth control function required for a relatively greater displacement for moving a focal point of light to a recording or reproducing-desired layer.
In order to meet the demands, a conventional focal depth controlling micro actuator includes two cylindrical coils arranged overlapping with each other on a concentric circle. The pair of coils generate an electromagnetic force by interaction, and a focal position is controlled by moving a lens focusing light in a light axial direction by using the electromagnetic force.
However, the conventional focal depth controlling micro actuator is disadvantageous in that it can be hardly implemented as a compact size and has excessive power consumption, so it is not suitable to be adopted for a small-size information storing device.